The present invention relates to multiple-compressor refrigeration systems and, more particularly, to multiple compressor refrigeration systems in which one or more of the compressors are selectively operated in response to varying system load requirements.
Large-scale commercial refrigeration systems such as those employed in supermarkets typically employ a plurality of compressors in a refrigeration circuit to compress the system working fluid. The refrigeration circuit includes a system condensor which receives the compressed working fluid from the compressor and a plurality of remotely located refrigerated cases or enclosures which receive the condensed working fluid from the system condenser and pass it through an expansion valve or other expansion device and an evaporator within the refrigerated enclosure to chill the space within the enclosure. Typically, the display enclosures include meat cases, beverage coolers, frozen food cases, ice chests, and the like. After the working fluid is passed through the evaporator, the expanded refrigerant is than returned to the compressors through a return or suction line where the cycle is repeated. As is well known in the art, the refrigeration load requirements for these systems can vary greatly depending upon the ambient temperature, the quantity of merchandise in the refrigerated enclosures, the loading of additional room-temperature merchandise into the enclosures, and the removal of chilled merchandise from the enclosures. Because of the widely varying load requirements, most large-scale refrigeration systems utilize a plurality of compressors with one or more of the compressors operated in response to system load requirements. For example, during light load periods, only one of the available compressors may be in operation; conversely, during heavy load periods, all the compressors may be in operation.
In most multiple-compressor systems, the compressors are controlled in response to system return line or suction pressure. In some systems, the individual compressors are provided with a pressure-responsive transducer at the suction inlet. Typically, the pressure controllers for the various compressors are set at successively higher cut-in pressures so that as the suction pressure rises, successive compressors will cycle on to cause the desired increase in compressor capacity and a consequent reduction in suction pressure to a preferred limit. As the suction pressure drops in response to additional compressor capacity coming on line, the last-on compressor is cycled by its transducer to the off state. Other refrigeration systems use a single pressure responsive sequencer which provides multi-stage control of the various compressors. This type of controller is typically connected to the suction side manifold and is electrically connected to each compressor in the system. The multi-stage sequencer automatically cycles on additional compressors in response to increases in suction line pressure and cycles the compressors off as suction line pressure diminishes.
Both types of mechanical controllers generally provide adequate suction line pressure control, although there are several drawbacks to these controllers from a commercial standpoint. In refrigeration systems, it is generally desirable to maintain the suction side pressure within a relatively narrow bandwidth to thereby maintain the evaporator temperature in a directly related temperature bandwidth. Mechanically responsive pressure controllers, by virtue of their mechanical structure, generally can not provide a cut-in/cut-out pressure difference or less than 5 psi. In those systems in which three or four compressors are utilized with each compressor set for cut-in at successively higher pressure, it is not uncommon for suction pressure to vary in a 15 psi range. In addition to this drawback, pressure responsive sensors respond to both short-term transient changes in suction pressure as well as longer term changes. Accordingly, a short-term transient increase in suction pressure can cause the starting of a disproportionally large amount of compressor capacity resulting in oscillations in the suction pressure and unnecessary compressor cycling.
Another disadvantage of the above-described mechanical systems is that the compressors are cycled solely in response to suction line pressure and, thus, are cycled even in the event that all the refrigeration cases are at their design temperature. This condition can arise, for example, when open refrigerated cases are being operated in store ambients lower than that for which the system was designed. In this case, continued control of compressor capacity in response to suction pressure can result in superfluous and inefficient compressor utilization, and in lower than desired product temperatures.